(1) Field of the Invention
The present invention relates to threaded fasteners and more specifically to a means for keeping threaded fasteners in position when subjected to vibration or shock and particularly to a set screw which may be tightened and loosened numerous times without destroying the set screw.
(2) Description of the Prior Art
It is well known that threaded fasteners loosen over time when the part to which the fastener is affixed is subjected to shock or vibration. This is particularly true of screw type fasteners used on devices with moving parts or in conjunction with motors or vehicles. To prevent this loosening, locking fasteners are available to hold a threaded fastener in place until it is purposefully loosened.
Set screws are especially prone to loosening because of vibration or shock. In many cases, a set screw is used to secure a collar to a D-shaped shaft by tightening the set screw in an aperture in the collar to apply pressure to the flat side of the shaft. The set screw is subjected to shock each time the shaft is started and vibration while the shaft is rotating. Unlike ordinary cap screws, set screws do not usually have a cap because a cap limits the distance the fastener can be tightened into an aperture and a protruding cap on a rotating part can damage equipment and personnel. Washers cannot be used in many of these applications because the use of washers requires the presence of a cap to retain the washer against the part.
Alternate methods are available for locking a set screw to a part to prevent the screw from loosening. These methods include staking the set screw to the part, and providing a substance in or with the set screw that binds the threads of the set screw to the threads of the part. Problems are apparent with each of the above solutions. Staking the set screw to the part requires prick punching the boundary between the set screw and the part. This method creates an unsightly surface on the part and, over multiple lockings, damages both the set screw and the part. Using a binding substance with the set screw can destroy the set screw and clog threads in the part. Furthermore, fasteners containing a binding substance cannot be reused.
Many types of prior art locking fasteners exist, but only a few of these fasteners are designed to lock when the end of the fastener contacts the end of a threaded aperture. For example, U.S. Pat. No. 2,900,863 discloses a two piece expansion bolt with a cylindrical shank and an expansion plug. When the shank of this fastener is driven into an aperture, the expansion plug acts to expand the lower end of the fastener against the walls of the aperture. Some disadvantages of this prior art invention are that the fastener is not easily removable from the aperture, the shank and expansion plug are two separate pieces, and the fastener is not designed for use in a threaded aperture.
Two other basic locking fastener designs are disclosed by U.S. Pat. Nos. 4,411,570 and 4,976,577. These patents disclose hollow, threaded, locking fasteners having a plunger inserted in the head of the fastener and driven into the fastener to expand the lower end of the fastener against the part. These inventions have the disadvantage that a special tool must be used to remove the fastener from the aperture. No provision is made for retaining the plunger in the fastener during insertion or storage. Furthermore, the head of the fastener must be near the surface of the part to allow the plunger to be driven into the fastener.